Method for making an impregnated ceramic material

ABSTRACT

A method for impregnating a shaped ceramic material with an impregnation liquid, comprising the steps of placing the shaped ceramic material in the impregnation liquid, and applying an impregnation pressure to the impregnation liquid through a pressure transfer liquid so that the shaped ceramic material is impregnated with the impregnation liquid.

BACKGROUND OF THE INVENTION

This invention relates to a method for impregnating a ceramic material such as a carbonaceous material or carbon material with an impregnation liquid and a product produced thereby.

A carbonaceous material is a typical example of various ceramic materials used as a structural material or a material for machines. Such a carbonaceous material is a good thermal and electric conductor has an excellent thermal resistance. In particular, its strength does not decrease at high temperatures.

On the other hand, the carbonaceous materials are porous. Thus, they do not have a good mechanical strength. Also, they are easily oxidized and therefore consumable.

In view of the foregoing, many researchers have tried to obtain a high density carbonaceous material which maintains it excellent characteristics for a long period of time. For example, dense particles are packed with a high density.

In another conventional method, pores of a carbonaceous material are impregnated with an impregnation liquid such as pitch. For instance, a shaped carbonaceous material and an impregnation liquid are set in a container. Pressurized air is applied directly to the impregnation liquid in such a way that the pores of the carbonaceous material are impregnated with the impregnation liquid to thereby produce a high density carbonaceous material. The impregnation liquid is heated at 80° C. or more. The pressure of the pressurized air is 20 Kg/cm² or less. In such a conventional method, however, the density of the carbonaceous material is not sufficiently high. The impregnation is not satisfactory.

SUMMARY OF THE INVENTION

The object of the invention is to provide a sufficiently high density ceramic material such as a carbonaceous material and a method of making the same.

According to the invention, an impregnation pressure is applied via a pressure transfer liquid to an impregnation liquid so that pores of a ceramic material are impregnated with the impregnation liquid.

Examples of the pressure transfer liquid are water, ethylene glycol, oil and alcohol.

The best example of a shaped ceramic material is a shaped carbonaceous material, but the invention is not limited thereto. Preferred examples of products made of the ceramic material are a graphite crucible for use in an aluminium deposition method or a CZ (Czochralski) method, a heater, a susceptor base material, a liquid-phase growth boat and a plasma CVD electrode.

Examples of the impregnation liquid are a thermosetting resin such as furfuryl alcohol, resin and tar pitch.

The impregnation pressure is preferably 20 Kg/cm² or more and for the best results 50 Kg/cm² or more.

A resilient material such as a rubber is disposed between the impregnation liquid and the pressure transfer liquid when the shaped ceramic material is impregnated with the impregnation liquid. For instance, a rubber press method is employed. Although it can be carried out at a room temperature, preferably a carbonaceous material is impregnated with tar pitch at a temperature of 100° C. to 280° C.

Preferably, the impregnation liquid has a viscosity of 300 C.P.(centipoise) at 20° C.

According to the invention, the impregnated carbonaceous material has a bulk density of 1.5 to 2.0 g/cm³, a specific resistance of 500 to 1900 micro-ohm-cm, a bending strength of 300 Kg/cm² or more, an apparent or open porosity of 0.1 to 7.0%, and a true specific gravity of 2.10 or less.

As the bulk density ranges between 1.5 g/cm³ and 2.0 g/cm³, if a crucible is made of the carbonaceous material, the surface thereof is not rough, and the material can be prevented from being graphitized too much. The bonding among the carbon particles is sufficiently maintained because the bending strength is 300 Kg/cm² or more. As the carbonaceous material has a specific resistance of 500 to 1900 micro-ohm-cm, the carbon particles can be prevented from being crystalized too much so as to remain extremely flexible. Also, the heating value does not become too large, so that temperature control is easy. Further, because the carbonaceous material has an apparent porosity of 0.1 to 7.0%, the pores among the carbon particles will not become so large as to decrease the bonding strength. For example, a crucible made of such a carbonaceous material is difficult to crack. Also, the carbon particles can be prevented from dislodging when silicon wafers for a semiconductor device are processed in a plasma CVD apparatus. If the true specific gravity is more than 2.10, the carbon particles are apt to be crystalized too much whereby the carbonaceous material lacks proper flexibility. In such a case, the service life decreases.

Accordingly, the present invention provides a carbonaceous material or carbon material having a bulk density of 1.5 to 2.0 g/cm³, an apparent porosity of 0.1 to 7.0%, and an air permeability ranging between 0 and 1.0×10⁻⁴ ml cm/cm².sec.cm.

Substantially no open pores are present in the carbonaceous material, so that it is not air permeable. Thus, spalling resistance is excellent.

When the air permeability is less than 1.0×10⁻⁴ ml. cm/cm².sec.cm, a low density fine carbon can be formed in the pores of the carbonaceous material. As a result, it is possible to decrease the volume of the gas which is produced from a graphitized crucible when pulling silicon crystals. On the other hand, the apparent porosity of 0.1 to 7.0% is effective for preventing spalling of a crucible.

Preferably, prior to applying the impregnation pressure to the impregnation liquid, a negative pressure is applied to the shaped material so that any gas remaining in the shaped material can be discharged therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic section of a cold isostatic press machine for carrying out a method of this invention;

FIG. 2 is a graphic view showing relationships between the bulk density and the impregnation pressure in case of carbonaceous materials;

FIG. 3 is a graphic view showing relationships between the bulk density and the number of impregnations for carbonaceous materials; and FIG. 4 shows relationships between the bulk density and the impregnation liquid viscosity.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a cold isostatic rubber press type impregnation machine.

The pressure transfer liquid is water 2 contained in a pressure-tight container 1. The impregnation liquid is furfuryl alcohol 4 contained in a rubber container 3. A shaped carbonaceous material 5 is also contained in the rubber container 3. The rubber container 3 is designed to be completely closed in operation, but may be opened when desired. The pressure-tight container 1 is closed by a closure 6 while the rubber container 3 is immersed in the water 2. The rubber container 3 functions as a separator.

In operation, the inside of the rubber container 3 is evacuated to 30 torr for ten minutes by means of a vacuum pump (not shown). After that, the furfuryl alcohol 4 is poured into the rubber container 3 by means of a feeder (not shown), and then the water 2 is pressurized such that an impregnation pressure of 300 Kg/cm² or more is applied to the furfuryl alcohol 4 for about one minute whereby the shaped carbonaceous material 5 is impregnated with the furfuryl alcohol. The impregnated carbonaceous material is heated up to 200° C. at a heating rate of 10° C./hour and further heated up to 950° C. at a heating rate of 8° C./hour. Finally, it is subjected to a purification treatment at 2,300° C.

EXAMPLES

Table 1 shows the results Examples 1 to 4 in which four shaped carbonaceous materials were impregnated with furfuryl alcohol.

The carbonaceous materials of the Examples 1 to 4 shown in Table 1 were impregnated by means of the impregnation apparatus shown in FIG. 1 and found to have the characteristics shown in Table 2.

For purposes of comparison, the shaped carbonaceous materials of Examples 2 and 3 in Table 1 were impregnated with furfuryl alcohol at five impregnation pressures of 1, 50, 500, 1000 and 1500 Kg/cm². The bulk density/impregnation pressure relationships are shown in FIG. 2. Where the impregnation pressure was 1 Kg/cm², many inner pores of the carbonaceous material were not impregnated with the furfuryl alcohol so that the bulk density was not sufficiently increased. At 50 Kg/cm² or more, the bulk density was remarkably increased.

FIG. 3 shows the relationships between the number of impregnations and the bulk density. The impregnation pressure was 1500 Kg/cm².

As can be seen from FIG. 3, in a conventional low-pressure pitch impregnation method, in the case of one impregnation, the bulk density increases only by several percent, and after that it does not efficiently increase even if an impregnation-firing cycle is repeated. Using high-pressure in the impregnation method of the invention, the impregnation efficiency is improved. In the case of two impregnations, the bulk density increases first to 1.91 g/cm³ and second to 1.96 g/cm³.

Table 3 shows experimental results for six comparative examples in case of graphite crucibles in comparison with a crucible made of a carbonaceous material according to the invention. The comparative examples are out of the scope of the invention with regard to at least one parameter among bulk density, specific resistance, bending strength, apparent porosity, true specific gravity and air permeability. The number of applications of the crucibles in use remarkably differs between the examples of the invention and the comparative examples. In this regard, the number of applications of the crucibles means the number thereof until cracking takes place.

In the experiments, a pure silicon of 35 Kg was melted in the above-described crucibles, and a silicon crystal having a diameter of 5 inch and the crystal orientation (100) was repeatedly pulled at a speed of about 1 mm/min. The volume of produced or discharged gas was measured at a room temperature through 950° C.

FIG. 4 shows the relationships between the viscosity of impregnation liquid and the bulk density The impregnation pressure was 1,500 Kg/cm². The shaped carbonaceous materials of Examples 2 and 3 in Table 1 were impregnated with polymerized furfuryl alcohol with viscosities of 30, 200, 250, 300, 350, 700 C.P. at 20° C.

As can be seen from FIG. 4, in case of a low viscosity such as 30 C.P. at 20° C., a large volume of the polymerized furfuryl alcohol is volatilized until a solid is formed. It cannot be expected to increase the bulk density. This is supported by the data in Table 4. The bulk density does not remarkably change between 300 C.P. and 700 C.P. at 20° C. Thus, the viscosity of the impregnation liquid is preferably at least 300 C.P. at 20° C.

                  TABLE 1                                                          ______________________________________                                                           Example No.                                                                    1    2       3      4                                        ______________________________________                                         Bulk density (g/cm.sup.3)                                                                          1.6    1.7     1.8  1.9                                    Bending strength (Kg/cm.sup.2)                                                                     300    450     500  600                                    Specific resistance 2500   1400    1150 1050                                   (micro-ohm-cm)                                                                 Apparent porosity (%)                                                                              18     12      8    8                                      True specific gravity                                                                              2.15   2.14    2.12 2.11                                   Air permeability    6.5    2.5     1.0  1.0                                    (× 10.sup.-4 ml · cm/cm.sup.2 · sec ·         cm)                                                                            Discharged gas volume (ml/100 g)                                                                   10.5   11.0    8.0  9.0                                    ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                           Example No.                                                                    1    2       3      4                                        ______________________________________                                         Bulk density (g/cm.sup.3)                                                                          1.7    1.8     1.9  2.0                                    Bending strength (Kg/cm.sup.2)                                                                     350    500     600  700                                    Specific resistance 1900   1200    1100 1000                                   (micro-ohm-cm)                                                                 Apparent porosity (%)                                                                              7      6       1    0.1                                    True specific gravity                                                                              2.05   2.08    2.02 2.00                                   The number of applications                                                                         69     79      77   85                                     of a crucible                                                                  Air permeability    0.8    0.7     0.4  0.1                                    (× 10.sup.-4 ml · cm/cm.sup.2 · sec ·         cm)                                                                            Discharged gas volume (ml/100 g)                                                                   6.5    5.0     1.0  1.6                                    ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                                      Comparative Example No.                                                        1    2      3      4    5    6                                    ______________________________________                                         Bulk density (g/cm.sup.3)                                                                     1.4    1.5    1.6  1.7  1.8  1.9                                Bending strength                                                                              400    250    400  350  500  350                                (Kg/cm.sup.2)                                                                  Specific resistance                                                                           5000   6000   8000 1800 1000 800                                (micro-ohm-cm)                                                                 Apparent porosity (%)                                                                         7      7      7    15   6    5                                  True specific gravity                                                                         2.05   2.04   2.10 2.10 2.15 2.20                               The number of applica-                                                                        7      3      15   13   16   13                                 tions of a crucible in use                                                     Air permeability                                                                              0.9    1.0    1.2  0.1  --   --                                 (× 10.sup.-4 ml · cm/cm.sup.2 · sec ·         cm)                                                                            Discharged gas volume                                                                         16.3   10.4   11.4 10.3 --   --                                 (ml/100 g)                                                                     ______________________________________                                    

                  TABLE 4                                                          ______________________________________                                         Viscosity  Volatilized Volume                                                                            Carbonization Rate                                   (C.P. at 20° C.)                                                                   (%)            after Hardened (%)                                   ______________________________________                                          30        24.4           37.6                                                 300        14.6           44.6                                                 700        13.6           45.8                                                 1350       12.9           47.4                                                 ______________________________________                                     

We claim:
 1. A method for impregnating a shaped, porous ceramic material with an impregnation liquid, said method comprising:providing a resilient separator having one surface in contact with the impregnation liquid and a pressure transfer liquid in contact with the opposite side of said separator, said separator thereby separating said impregnating liquid from said pressure transfer liquid; immersing the ceramic material in the impregnation liquid; applying an impregnation pressure to said pressure transfer liquid, said pressure being transferred through said resilient separator to said impregnation liquid to force said impregnation liquid into the pores of the shaped porous ceramic material.
 2. The method of claim 1, wherein prior to applying the impregnation pressure to the impregnation liquid, a negative pressure is applied to the shaped material so that any gas remaining in the shaped material can be discharged therefrom.
 3. The method of claim 2, wherein the impregnation liquid is tar pitch or a thermosetting resin.
 4. The method of claim 1, wherein the impregnation pressure is 20 Kg/cm² or more.
 5. The method of claim 1, wherein the impregnation pressure is 50 Kg/cm² or more.
 6. The method of claim 1, wherein a resilient separator is placed between the impregnation liquid and the pressure transfer liquid in such a way that the impregnation pressure is transferred via the pressure transfer liquid and the resilient separator to the impregnation liquid.
 7. The method of claim 1, wherein the pressure transfer liquid is water, ethylene glycol, oil or alcohol.
 8. The method of claim 1, wherein the resilient separator is made of rubber.
 9. The method of claim 1, wherein the separator is a rubber container.
 10. The method of claim 1, wherein the shaped ceramic material is impregnated with the impregnation liquid by a rubber press method.
 11. The method of claim 1, wherein the shaped ceramic material is impregnated at a temperature of 100° C. to 280° C.
 12. The method of claim 1, wherein the impregnation liquid has a viscosity of 300 C.P.(centipoise) or more at 20° C.
 13. The method of claim 1 wherein said separator is a container, wherein impregnation liquid and said ceramic material are placed in said container and said container is then closed and wherein the closed container is then immersed in the pressure transfer liquid.
 14. The method of claim 1 wherein said impregnation liquid contains tar pitch or a thermosetting resin.
 15. The method of claim 1 wherein said pressure transfer liquid contains no tar or thermosetting resin.
 16. The method of claim 1 wherein said impregnation liquid and said pressure transfer liquid are two different liquids.
 17. The method of claim 14 wherein the container is a rubber container. 